3198
Macromolecules 1990, 23, 3198-3200
in full agreement with the 13C NMR data recorded on such a sample where high-resolution spectra are obtained with a multiplicity of 2 at C1, C4, and C6. The explanation of the diffraction diagram of the initial Valoniu is less straightforward. From the analysis of the l3C NMR spectrum of Valonia cellulose, Atalla and VanderHart6s7J4 have demonstrated that Valonia was composed of two distinct crystalline phases: the dominant Icu and the minor IP. The above experiment has demonstrated that I@corresponds to a monoclinic phase. Therefore, the pure Ia diffraction diagram can be obtained by substracting the Ip fraction from this diagram. Such substraction is, however, not very accurate as one does not know the exact proportion of the I a and I@phases. Nevertheless, the Ia diffraction spectrum can be interpreted in at least two ways: (a) It may correspond to a two-chain triclinic cell with a = 0.954 nm, b = 0.825 nm, c = 1.036 nm, a = 90°, P = 57.0', and y = 96.6'. Such a cell would account for almost all the extra reflections arrowed in Figure 1D. (b) It may also correspond to an eight-chain triclinic cell similar to that of Honjo and Watanabe* with a = 1.584 nm, b = 1.644 nm, c = 1.036 nm, o = p = 90°, and y = 97.3'. Our favor goes toward the first hypothesis, but unless a pure Ia form is prepared or pure Ia diagram is obtained, it is difficult to give a clear answer on the unit cell or crystal structure of I a cellulose. One may speculate as to why a highly crystalline native cellulose sample such as Valonia is found in the form of such a composite crystalline structure. It is likely that such occurrence results from the cellulose biogenesis mechanism. During such a biosynthetic process, cellulose is synthesized, spun, and crystallized almost simultaneously at a temperature far below the glass transition or the melting point of cellulose. This undoubtedly leads to a structure where strains and stresses are built in. We suggest that the triclinic I a phase, which is more or less abundant depending on the specimen origin, corresponds to this strained crystalline form. The strains would be released when the tempertaure is raised, in particular, above the cellulose softening temperature, to give the relaxed monoclinic IP form. The crystalline composite features observed for native cellulose are not unique in the world of either "nascent" or deformed crystalline polymers. This had been well documented in the case of crystalline polyethylene where a triclinic phase was also found in nascent samples, especially those prepared at a very low t e m p e r a t ~ r e . ' Such ~ a triclinic phase is also found in stretched polyethylene as for instance in the gel spun samples.16 In both cases, the triclinic phase is metastable as opposed to the stable orthorhombic crystalline polyethylene. Upon annealing, the triclinic metastable form relaxes and disappears readily when the temperature of the specimen is brought in the vicinity of the melting point.17 This is very simi-
Size-Quantized, Semiconductor Particle Mediated Photoelectron Transfer in Ultrathin, Phosphonate-Functionalized,Polymer-Blend Membranes Size and dimensionality reductions of semiconductor particles result in altered mechanical, chemical, electrical, electrooptical, and magnetic properties, which could be profitably exploited in a variety of applications, includ0024-9297/90/2223-3198$02.50/0
lar to what happens in the present cellulose experiment. Acknowledgment. We thank Dr. H. Chanzy from CERMAV-CNRS, France, for valuable discussions during the writing of this manuscript. This work was supported by a Grant-in-Aid for Scientific Research from the Ministry of Education and Culture, Japan (Grant Nos. 63440013 and 01760133). References and Notes Meyer, K. H.; Misch, L. Helu. Chim. Acta 1937, 20, 232. Honjo, G.; Watanabe, M. Nature 1958, 181, 326. Gardner, K. H.; Blackwell, J. Biopolymers 1974, 13, 1975. Sarko, A.; Muggli, R. Macromolecules 1974, 7, 486. Atalla, R. H.; Gast, J. C.; Sindorf, D. W.; Bartuska, V. J.; Maciel, G. E. J . Am. Chem. SOC.1980, 102, 3249. Atalla, R. H.; VanderHart, D. L. Science 1984,223, 283. VanderHart, D. L.; Atalla, R. H. Macromolecules 1984, 17, 1465. In a slightly different explanation, Horii et al. (Macromolecules 1987,20,2117)proposed the nomenclature Ib/Ia to account for the complexity of the CP/MAS NMR spectrum. In this study, however, we prefer the Ia and 10 nomenclature as it corresponds to the first description of the two crystalline components of native cellulose.6 Belton, P. S.; Tanner, S. F.; Cartier, N.; Chanzy, H. Macromolecules 1989, 22, 1615. Chanzy, H.; Henrissat, B.; Vincendon, M.; Tanner, S. F.; Belton, P. s. Carbohydr. Res. 1987, 160, l . Hirai, A.; Horii, F.; Kitamaru, R. Macromolecules 1987, 20, 1944. Horii, F.; Yamamoto, H.; Kitamura, R.; Tanahashi, M.; Higuchi, T. Macromolecules 1987,20, 2946. Yamamoto, H.; Horii, F.; Odani, H. Macromolecules 1989,22, 4130. VanderHart, D. L.; Atalla, R. H. In The Structure of Cellulose; Atalla, R. H., Ed.; ACS Symposium Series 340; American Chemical Society: Washington, DC, 1987; 88. Smith, P.; Chanzy. H.; Rotzinger, P. J . Mater. Sci. 1987, 22, 523. Chanzy, €3.; Smith, P.; Revol, J.-F.; Manley, R. St. J. Polym. Commun. 1987, 28, 133. Pennings, A. J.; Zwijnenburg, A. J . Polym. Sci., Polym. Phys. Ed. 1979. 17, 1011. To whom all correspondence should be addressed. Present address: CERMAV-CNRS, B.P. 53X-38041, Grenoble Cedex, France.
Junji Sugiyama'Js and Takeshi Okano Department of Forest Products Faculty of Agriculture, T h e University of Tokyo Yayoi, B u n k y o - k u , Tokyo 113, J a p a n
Hiroyuki Yamamoto Fukui Technical College, Sabae, Fukui 916, J a p a n
Fumitaka Horii Institute f o r Chemical Research, Kyoto University, Uji, Kyoto 611, J a p a n Received January 3, 1990 Revised Manuscript Receiued March 29, 1990
ing solar energy conversion.3-5 Carefully controlled experimental conditions are required, however, for the preparation and stabilization of ultrasmall colloidal semiconductor particles. Inspired by the ability of commercially available Nafion membranes to incorporate colloidal semiconductors and we have developed functionalized, ultrathin, polymer-blend membranes (PBMs) as matrices for size-quantized semiconductor particles. Our work shows that miscible polymer blends provide an excel1990 American Chemical Society
Communications to the Editor 3199
Macromolecules, Val. 23,No. 1.2, 1990
510
500
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5ao
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ANOLE OF INCIDENCE Figure 1. Plots of reflectivity for the empty (Ro:A) "d PSP-CA blend membrane (RPBM.: 0 )against the angle of incidence. Intersection of curves Ro with R p e gave ~ the Brewster angle. The inset shows typical interference infrared bands.
BW
4
9
8
w
3
IAvpO*inl M
Figure 2. Absorption spectra of a CdS particle containing (A) and CdS-free (B)PBM (ea. 400 A thick). A transmission elec-
tron microgram is shown in the inset. lent membrane medium for the present application since they can be easily tailored to contain micro- and submicrophases (clusters), which complex the metal ion precursor of the desired semiconductor particles. The subject of the present paper is the in situ generation of monodispersed, 75-100-A-diameter CdS particles in ultrathin PBMs (ca. 400 A thick) and their utilization to mediate photoelectron transfer to methylviologen. Ultrathin PBMs were prepared from the miscible polymer pair poly(styenephosphonate diethyl ester) (PSP; MW = 24 000; Tg= 11 "C) and cellulose acetate (CA; MW = 30 000; Ts= 185 "C). These polymers have been shown to interact well with each other, to form a homoand to be amenable for geneous phase with a single Tg, thin-film formation.'"12 Phase segregation and formation of 75-100-A, PSP-rich clusters in the CA-rich continuous phase of the ultrathin PBM can be induced by subjecting the membrane to a variety of conditions and/or casting it from the appropriate solvents @-dioxane, for example).'O In the PSP-CA blend system, the phosphoryl group (P 0)can chelate inorganic salts of transition elements and can also form strong hydrogen bonds with the residual hydroxyl groups of CA. The function of semicrystalline CA is to provide mechanical integrity for the ultrathin PBM. The PBM was prepared by spreading a solution of 0.1 wt % PSP-CA (1:l by weight) in anhydrous dioxane over the entire surface (2 cm2) of a dust-free glass slide. The dioxane was allowed to evaporate slowly for 48 h in an
-
0
2
4
6
I
IO
107MVl M Figure 3. Emission spectra of a CdS-containing ca. 400-Athick PBM in the presence of increasing amounts of MV2+ in the solution soaking the membrane. h, = 400nm (inset). The resulting plot of relative fluorescence intensity vs [MV2+lM indicates a linear Stem-Volmer behavior. enclosed container and w&ssubsequently separated from the glass slide by water. The PBM was then picked up from the water surface by a filter paper and dried. In general, this procedure was found to be adequate for the formation of continuous CA-rich PBMs, which contained 75-100-A-diameter, PSP-rich clusters. Cadmium ions were incorporated into the PBM by floating the membrane on an aqueous (1.0 X l(r3M) Cd(NOd2 solution for 12 h (on each side). The infrared spectra (KBr plate) of the phosphoryl stretching band revealed a significant downfield shift from 1251 to 1231 cm-I, which is in accord with the expected change accompanying inorganic salt complexation to the phosphoryl group. Particles of CdS were formed in the PBM by exposing the membrane to H2S. The cadmium ion containing PBM was floated on the surface of the Cd(NO& solution and placed in an air-tight vessel into which 200 CL of Hi3 was slowly infused. Penetration of H2S into the membrane was ascertained by the visual observation of light yellow domains on both sides of the membrane surface. In contrast, opaque yellow coloration was observed on the water surface surrounding the PBM (which subsequently led to similar coloring of the subphase and t o eventual precipitation of large CdS particles). Subsequent to the infusion of all of the HzS, the CdS-containing PBM was removed from the surface of the Cd(N03h solution, repeatedly rinsed in dust-free water, and used for further characterization and electron-transfer measurements. No leaching of CdS particles could be detected over periods of 10 days on soaking the CdS-containing PBMs in water. The thicknesses of PBMs were determined by combined refractive index and infrared interference fringe (AX) measurements. Optical reflectivity measurements followed previously described experimental proced~res.'~ The monochromatic reflectivity for the p-polarized incident light (514.5 nm, 50" Ar+ laser) was evaluated as a function of the incident angle, eo, for the uncoated (Ro) and the PBM-covered (RPBM) glass slide (Figure 1). The angle a t which the 8, vs Ro curve crossed the 80 vs RPBM curve (55.2") was equated with the Brewster angle due t o the PBM, Be. The corresponding refractive index, n, was assessed to be 1.439 from n=tan$
(1)
3200 Communications to the Editor
Macromolecules, Vol. 23, No. 12, 1990
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0.96
0.72 0
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IRRADIATION TIME. mlnnloi
4 0.24
0.00 1 300
I
400
600
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700
800
WAVELENGTH, nm Figure 4. Absorption spectra of a CdS-containing ca. 400-Athick PBM in a n aqueous, degassed solution, which contained 4.0 X 10-4 M MV2+ ( p H = 7.2) and 0.01 % benzyl alcohol (v/v) as a function of increasing irradiation time by a 200-W Hg-Xe lamp using a 400-nm cutoff filter. Increasing irradiation (time) resulted in the increasing absorbances with maxima at 397 and 604 nm. T h e amount of MV*+ produced (taking t ~ w =+ 13 700 M-* cm-1 at 603 nm) as a function of irradiation time is shown in the inset.
Infrared interference fringe measurements yielded AA Values for the PBM (see the inset in Figure 1) from which the membrane thickness, d , was calculated by means of d = dA/2n (2) Substitution of the appropriate values into eq 2 led to 398 8, for the thickness of the PBM. Absorption spectral4 clearly revealed the presence of CdS particles in the PBM (Figure 2). The observed band edge, 475 nm, corresponds to 70-8, diameter particle^.^ Transmission electron micrograms (taken on a JEOL JEM2000 EK, 120-keV instrument) confirmed the presence of 75-100-A-diameter CdS particles (see inset in Figure 2). Importantly, the particles are well separated and uniform. Excitation of the CdS-containing PBM a t 400 nm resulted in a broad structureless band with an emission maximum at 560 nm (Figure 3).15 Addition of increasing concentrations of methylviologen (MV2+),a known electron acceptor, to the aqueous solution bathing the CdS-containing PBM (on a quartz support) progressively decreased the fluorescence and resulted in a linear Stern-Volmer plot (Figure 3). Illumination with visible light (Aex > 400 nm) of the CdS-containing PBM in an aqueous, degassed solution that contained 4.0 X M MV2+ (pH = 7.2) and 0.01% benzyl alcohol (v/v) led to the development of a blue color (Figure 4). These results are explicable in terms of forming conduction-band electrons and valence-band holes in the band-gap excitation of the CdS: CdS
hu
eCB-
+ hVBt
(3)
Recombination of a small fraction of the charged species results in fluorescence:
-
e,-< + MV2' MV" (5) and thus decreases the fluorescence yield (i.e., quenching occurs). Benzyl alcohol is a sacrificial electron donor whose function is to remove hvB+ and, thus, increase the yield of MV'+ and, hence, its absorbance (A, = 395 and 604 mm) as a function of irradiation (Figure 4). Electron transfer has been shown to be more efficient from smaller than from larger CdS particles. The importance of PBMs is that they provide inert matrices for monodispersed, size-controlled semiconductor particles and, hence, allow the intimate investigation of the effects of size quantization on electron transfer.
eCB-+ hVB+ hv (4) MV2+,an electron acceptor, competes with reaction 4 by
Acknowledgment. Support of this work by the Center for Membrane Engineering and Science is gratefully acknowledged. We thank Bruno Guillaume (Department of Chemistry, Syracuse University) for aiding in the PBM thickness determinations and Dr. R. R. Hanna (Ultrastructure Department of the State University of New York College of Environmental Science and Forestry) for assistance in obtaining TEMs. References and Notes (1) Department of Chemistry, Syracuse University, 1-014 Center for Science and Technology, Syracuse, NY 13244-4100. (2) Chemistry Department and the Polymer Research Institute, State University of New York College of Environmental Science and Forestry, Syracuse, NY 13210-2786. (3) Fendler, J. H. J . Phys. Chem. 1985,89, 2730. (4) Fox, M. A.; Chanon, M. In Photoinduced Electron Transfer; Elsevier: Amsterdam, The Netherlands, 1988. (5) Henglein, A. Top. Curr. Chem. 1988, 143, 113. Brus, L. A. J . Phys. Chem. 1986, 90, 2555. Andres, R. P.; Averback, R. S.; Brown, W. L.; Brus, L. E.; Goddard, W. A.; Kaldor, A.; Louie, S. G.; Moskovits, M.; Percy, P. S.; Riley, S. J.; Siegel, R. W.; Spaepen, F.; Wang, Y. J . Mater. Res. 1989,4, 704. (6) Meissner, D.; Memming, R.; Kastening, B. Chem. Phys. Lett. 1983, 96, 34. (7) Krishnan, M.; White, J. R.; Fox, M. A.; Bard, A. J. J. Am. Chem. SOC.1983, 105, 7002. Mau, A. W. K.; Huang, C. B.; Kakuta, N.; Bard, A. Z.; Campion, A.; Fox, M. A,; White, M.; Webber, S. E. J . Am. Chem. SOC. 1984,106,6537. (8) Kuczynski, J. P.; Milosavljevic, B. H.; Thomas, J. K. J. Phys. Chem. 1984, 88, 980. Milosavljevic, B. H.; Thomas, J. K. J. Am. Chem. SOC. 1986,108, 2513. (9) Makhmadmurodov, A.; Bruzdkov, Y. A.; Savinov, E. N.; Parmon, V. N. Kinet. Catal. 1986,27, 121. (10) Gardiner, E.; Cabasso, I. Polymer 1987,28, 2052. (11) Sun, J.; Cabasso, I. J . Polym. Sci., Part A: Polym. Chem. 1989, 27, 3985. (12) Sun, J.: Frisch. H. L.: Cabasso. I. J . Polvm. Sci.. Part B: Polvm. Phys. 1989, 27, 2657. (13) Zhao, X.K.; Baral, S.: Rolandi, R.: Fendler. J. H. J . Am. Chem. SOC.1988, 110, 1012. (14) Absorption spectra were taken on a Hewlett-Packard 8450 A diode-array spectrophotometer. (15) CdS particles containing PBMs (on a quartz substrate) were securely positioned a t a 45O angle to the incident excitation source and detector. Emission spectra were recorded on a Tracor Northern 6500 diode-array spectrophotometer.
Youxin Yuan,' Israel Cabasso,*J and Janos H. Fendler'vl Center for Membrane Engineering a n d Science Syracuse University, Syracuse, New York 13244-41 00 Received J a n u a r y 15, 1990 Revised Manuscript Received April 4, 1990
